1. Field of the Invention
The present invention relates to a method for optimizing plasma processing. More particularly, the present invention relates to a method for optimizing a gap for plasma processing to improve processing uniformity and throughput.
2. State of the Art
Plasma processing devices are widely used for chemical vapor deposition of materials on substrates and etching of substrates by supplying process gas to a plasma processing chamber and applying a radio frequency (RF) field to the gas. As shown in FIG. 1, a typical plasma processing device includes a chamber 100 enclosing a lower electrode 104, which supports a substrate 140, and an upper electrode 102. The lower electrode 104 is powered by an RF power source 114, and the upper electrode 102 is powered by an RF power source 112. Reactant gas supplied into the processing chamber is excited into a plasma by RF power applied through the electrodes 102 and 104, and the excited plasma causes deposition and/or etching on the substrate 140. Substrates typically processed in such a device include semiconductor wafers and flat panel displays.
Examples of a plasma processing device include the LAM 4420 Polyetcher, which employs a mechanical clamp to hold the substrate on the lower electrode, and the LAM 4420XL Polyetcher, which employs an electrostatic chuck (ESC) to hold the substrate in place via an electrostatic field.
It is important to maximize yield and throughput in a plasma processing device. Various processing parameters affect yield and throughput, including processing pressure. Plasma processing devices often include turbomolecular pumps used in combination with load locks to maintain a desired pressure for the plasma processing chamber. For example, FIG. 2 illustrates a turbomolecular pumped plasma processing device. Referring to FIG. 2, the turbomolecular pumped plasma processing device includes a main plasma processing chamber 100, load locks 110 and 120, and a turbomolecular pump 130. Substrates are input into the load lock 110, processed through the main chamber 100, and output through the load lock 120. The load locks 110 and 120 employ dedicated pumps to increase the substrate transfer time into and out of the main plasma processing chamber 100, thus optimizing throughput of the plasma processing device. The turbomolecular pump 130 allows lower operating pressures at higher pumping rates which not only complements throughput but also allows a wider range of possible pressure settings for plasma processing. Thus, turbomolecular pumped plasma processing devices have proven effective for improving yield and enhancing throughput.
Another processing parameter affecting throughput and yield in a plasma processing device is the gap between the upper electrode and the lower electrode. Different gap settings result in different concentrations of excited plasma above the substrate. Thus, the processing rate, and hence the throughput, vary depending on the gap setting. The concentration of excited plasma can also vary across the surface of the substrate, depending on the gap setting. Therefore, the gap setting also affects processing uniformity, therefore affecting the yield.
Conventionally, gaps are set to optimize the processing rate, without concern for uniformity. Since the best gap setting for the processing rate can not be the best gap setting for uniformity, this conventional method of setting the gap does not produce an optimal yield.
In addition to processing pressure and the gap between the upper electrode and the lower electrode, there are other factors that affect processing yield and throughput, including processing gas flow and RF power. If the plasma processing device is not producing sufficient yield or not operating at an acceptable throughput, the problem can be that one of the processing parameters is not properly set. Often, it is difficult to determine which parameter is not set properly.
There is thus a need for a method to determine an optimal gap that provides both improved uniformity and throughput. There is also a need for a method for determining if a gap in a plasma processing device is properly set.